Doors can get old. They can develop holes from being kicked, no longer function as they should, or you may just decide to replace the hollow core door with a solid door. Unless you have a house with a standard door size, you will probably have to trim the new door down to size. This is something that you can do yourself with some tools you may already have in your workshop.
Pick out the door
There are many sources for doors: at your local building supply company, surplus building supply stores which specialise in items removed from houses slated for demolition, on the side of the road, in yard sales and more. The door can be plain, with door panels, with a finish or without.
Remove the existing door
Remove the existing door. Pins in the hinges should slip out. If they do not come easily, gently remove them by inserting the blade of a screwdriver beneath the pin and rap on the handle of the screw driver with a hammer to drive pin out. It will probably not be necessary to remove the door moulding.
Mark the new door
Lay the old door on top of the new door spanning a set of saw horses. Line up the old door so that the new door is even - this is important if the doorway is oddly shaped or not square (can be the case in older construction). You want to give the impression of the door hanging properly. Trace around the old door with a pencil, creating the lines which you will cut to fit the door to size.
Cut the new door
Cut the new door using a combination of a circular saw and a straight-edge guide. This will help you ensure that you cut nice, straight lines. If you need to cut 3/16 inch or less, then you can use a plane.
Lay the old door back on top of the new door. Using a combination square as a guide, mark the beginning and end of each hinge mortise. A utility knife works well for this application. Stand the new door on its end and use a chisel to cut the hinge mortises. Make sure to keep the angle of the chisel shallow so that it doesn't cut too deeply.
Set the hinges
Lay the hinge in the hinge mortise and check its depth. If it fits properly, then you can screw the hinge in place. If the mortise is too shallow, use the chisel to make the mortise deeper. Dry fit to check the depth, then screw in place. Repeat this step for all the hinges.
Drill the hole for the door knob and use the enclosed template with the new lockset to install it or reuse the old one.